DEFINITIVE METHOD CERTIFICATION OF CLINICAL ANALYTES IN LYOPHILIZED HUMAN SERUM - NIST STANDARD REFERENCE MATERIAL (SRM) 909B

The National Institute of Standards and Technology (NIST) has develope d several Standard Reference Materials (SRMs) based on human serum. NI ST SRM 909b, Human Serum, is a lyophilized human serum material with c oncentrations for seven organic and six inorganic analytes at two leve ls certified solely by definitive methods (DMs). This material provide s the vehicle by which high precision, high accuracy measurements made with DMs at NIST can be transferred through the measurement hierarchy to other laboratories. Isotope dilution gas chromatographic-mass spec trometric (GC-IDMS) methods were applied to measure cholesterol, creat inine, glucose, urea, uric acid, triglycerides, and total glycerides. Thermal ionization isotope dilution mass spectrometry (TI-IDMS) was us ed for determination of lithium, magnesium, potassium, calcium, and ch loride. In addition, chloride was determined by coulometry. providing a comparison between two DMs. Sodium, which lacks a stable isotope tha t would permit isotope dilution mass spectrometric (IDMS) measurement, was determined by gravimetry. SRM 909b includes certified values for total glycerides and triglycerides, which were not certified in the pr evious lot of this material (SRM 909a). Improvement in uniformity of v ial fill weight in the production of SRM 909b resulted in smaller cert ified uncertainties over previous freeze-dried serum SRMs. Uncertainti es at the 99% level of confidence for relative expanded uncertainty (% ) for certification of the organic analytes on a mmol/L/g basis ranged from 0.44% for urea (level II) to 5.04% for glucose (level II). (In-h ouse studies have shown glucose to be a relatively unstable analyte in similar lyophilized serum materials, degrading at about 1%, per year. ) Relative expanded uncertainties (99% C.I.) for certification of inor ganic analytes on a mmol/L/g basis ranged from 0.25% for chloride (lev el I) to 0.49% for magnesium (level II).